A car bomb on Tuesday rocked Beirut's southern suburbs, wounding at least 15 people in the stronghold of Lebanon's Shiite Hezbollah movement, television reports and a military source said.
The blast was quickly linked by observers to the civil war raging in neighbouring Syria, where Hezbollah fighters have joined President Bashar al-Assad's forces in facing down a revolt by mainly Sunni rebels.
"A car bomb exploded near a commercial cooperative called the Islamic Cooperation Centre in Bir al-Abed," which lies in the heart of Hezbollah's Beirut stronghold, the military source said.
He gave a toll of 15 people wounded, one of them badly.
The densely populated Bir al-Abed neighbourhood is home mainly to Shiite Muslims.
Several broadcasters, among them Hezbollah's Al-Manar, showed firefighters fighting several fires while large clouds of black smoke billowed into the sky.
A witness told AFP the explosion was "huge".
"Everyone started panicking. Everyone was running left and right" after the blast, said Carole Mansour, who owns a shoe shop near the affected area.
"The smoke was so high," Mansour told AFP, adding that Hezbollah members dressed in civilian clothing were quick to deploy around the site of the blast.
"I started following the sounds of the screams of people. My employees ran to the site to try to see what was happening because they have relatives there," she added.
"I can't believe someone would do this on the first day of Ramadan," said Mansour, referring to the Muslim holy fasting month.
Some Shiites started their fasting on Tuesday, although other Shiites and Sunnis will begin fasting either Wednesday or Thursday.
Officially neutral in Syria's conflict, Lebanon is however deeply divided into pro- and anti-Assad camps.
Hezbollah and its allies back Assad, who adheres to the Alawite offshoot of Shiite Islam, while the Sunni-led opposition supports rebels seeking his ouster.